Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element units including optical elements, such as lenses and mirrors etc., arranged in the exposure light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. In particular with mainly refractive systems working at a wavelength in the so-called vacuum ultraviolet (VUV) range (e.g. at a wavelength of 193 nm), such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually include an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Due to the ongoing miniaturization of semiconductor devices there is, however, it is desirable to enhance resolution of the optical systems used for fabricating those semiconductor devices. This pushes the desire for an increased numerical aperture (NA) and increased imaging accuracy of the optical system.
One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been taken using light in the extreme ultraviolet (EUV) range, typically using wavelengths ranging from 5 nm to 20 nm, in most cases about 13 nm. In this EUV range it is not possible to use common refractive optics any more. This is due to the fact that, in this EUV range, the materials commonly used for refractive optical elements show a degree of absorption that is too high for obtaining high quality exposure results. Thus, in the EUV range, reflective systems including reflective elements such as mirrors or the like are used in the exposure process to transfer the image of the pattern formed on the mask onto the substrate, e.g. the wafer.
The transition to the use of high numerical aperture (e.g. NA>0.4 to 0.5) reflective systems in the EUV range leads to considerable challenges with respect to the design of the optical imaging arrangement.
One of the desired accuracy properties is the accuracy of the position of the image on the substrate, which is also referred to as the line of sight (LoS) accuracy. The line of sight accuracy typically scales to approximately the inverse of the numerical aperture. Hence, the line of sight accuracy is a factor of 1.4 smaller for an optical imaging arrangement with a numerical aperture NA=0.45 than that of an optical imaging arrangement with a numerical aperture of NA=0.33. Typically, the line of sight accuracy ranges below 0.5 nm for a numerical aperture of NA=0.45. If double patterning is also to be allowed for in the exposure process, then the accuracy would typically have to be reduced by a further factor of 1.4. Hence, in this case, the line of sight accuracy would range even below 0.3 nm.
Among others, the above leads to very strict desired properties with respect to the relative position between the components participating in the exposure process as well as the deformation of the individual components. Furthermore, to reliably obtain high-quality semiconductor devices it is not only desirable to provide an optical system showing a high degree of imaging accuracy. It is also desirable to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical imaging arrangement components, i.e. the mask, the optical elements and the wafer, for example, cooperating in the exposure process is supported in a well-defined manner in order to maintain a predetermined spatial relationship between the optical imaging arrangement components and to provide minimum undesired deformation as well to provide a high quality exposure process.
To maintain the predetermined spatial relationship between the optical imaging arrangement components throughout the entire exposure process, even under the influence of vibrations introduced, among others, via the ground structure supporting the arrangement and/or via internal sources of vibration disturbances, such as accelerated masses (e.g. moving components, turbulent fluid streams, etc.), as well as under the influence of thermally induced position alterations, in imaging arrangements working either the EUV range or the VUV range, it is desirable to at least intermittently capture the spatial relationship between certain components of the optical imaging arrangement and to adjust the position of at least one of the components of the optical imaging arrangement as a function of the result of this capturing process. Similar applies to the deformation of at least some of these components of the optical imaging arrangement.
These active solutions, however, typically involve active systems including a large number of actuators and sensors etc. Heat dissipation of these components increasingly aggravates thermal issues arising in such imaging arrangements, especially if they are located inside the housing receiving the optical elements, e.g. inside the so-called lens barrel.
One particular issue that arises with heat dissipation is the fact that optical elements used in such optical imaging processes, typically, are made of comparatively expensive materials having a very low coefficient of thermal expansion in order to avoid distortion of the optical surfaces due to thermally induced deformation of the optical element. On the other hand, not least for cost reasons, the support structure of such optical element units is made of less expensive materials, typically having a noticeably higher coefficient of thermal expansion than the material of the optical element.
As a consequence, typically, some sort of parasitic deformation decoupling is provided (in at least one decoupling degree of freedom, typically several decoupling degrees of freedom) between the support structure and the optical element in order to at least partially compensate for such differences in thermal expansion between the support structure and the optical element. Such parasitic deformation decoupling, typically, is achieved via parts located in the force flow between the support structure and the optical element, which are sufficiently compliant in the decoupling degree(s) of freedom in order to absorb the differences in thermal expansion.
An issue that, among others, arises in this context is the fact that many materials allowing achievement of such deformation decoupling properties (i.e., among others, sufficient compliance to provide such deformation decoupling in certain degrees of freedom) have a coefficient of thermal expansion, which has a considerable mismatch with the coefficient of thermal expansion of the optical element (as is the case with most metal materials). Hence, despite their compliance, use of such materials leads to the generation of noticeable parasitic stresses at the interface with the optical element.
On the other hand, other materials with a matching coefficient of thermal expansion, in many cases, do not provide sufficient compliance to provide such parasitic deformation decoupling (as is the case, for example, with most ceramic materials). Hence, even a small mismatch between the coefficients of thermal expansion still leads to noticeable parasitic stresses at the interface with the optical element.
One material that provides, both, sufficient compliance in a sufficiently matching coefficient of thermal expansion is the so-called Invar material, an iron (Fe) nickel (Ni) alloy (sometimes also referred to as FeNi36 or 64FeNi). Here, however, the issue arises that Invar shows noticeable magnetostriction properties, i.e. dimensional alterations in response to changing magnetic fields, which results in imaging accuracy issues for optical elements located in close proximity to components generating changing magnetic fields. Such a situation may, for example, arise in the above optical imaging arrangements having a large numerical aperture NA, which in many cases involves an optical element located in close proximity to the moving wafer stage. Here, the actuators generating the motion of the wafer stage also generate noticeably fluctuating magnetic fields in the surroundings of the neighboring optical element. As a consequence, here, magnetostriction effects may lead to noticeable parasitic stresses at the interface with the optical element.